Cellular Respiration in Mammals

• Animals require energy to survive, grow, and multiply. This energy is supplied through the oxidative degradation of high-energy substrates (sugars, fats, proteins). This process recovers the free energy contained in their chemical bonds.
• Cellular respiration, a biochemical process that happens in the mitochondria inside living cells, obtains energy from food molecules.
• The body stores basic fuels like adipose tissue and glycogen, but has no oxygen reserves. Cells have a limited amount of ATP, and require a constant oxygen supply to the mitochondria to enable aerobic tissue metabolism.
• Multicellular organisms require the respiratory and circulatory systems since O2 and CO2 do not diffuse easily unlike unicellular organisms.

Breathing Types

• External respiration involves the exchange of gases between air and blood.
• Internal respiration involves the exchange of gases between blood and cells.
• Cellular respiration involves the use of O2 by cells for metabolism.

Mammalian Respiratory System

• The respiratory system comprises the airways and lungs.

Anatomy of the Respiratory Tract

Upper Airways

• Nasal cavities: hairs trap coarse dust, and the inner wall is lined with a vascularized mucous membrane with ciliated prismatic epithelium.
• Pharynx: It is the intersection of the air and digestive tracts, contains tonsils to capture microorganisms.
• Larynx: The organ of voice allows air to pass into the trachea, and the epiglottis closes during swallowing.
• Trachea: It carries air from the larynx to the bronchi; cartilaginous rings keep it open but allow the esophagus to dilate. Secretions from the prismatic stratified epithelium with caliciform mucous cells retain dust from inspired air, which is then pushed back by vibratory cilia.

Lower Airways

• Bronchi: Formed by the division of the trachea; branch into smaller bronchi.
• Bronchioles: Less than 1 mm in diameter, regulated by smooth muscle fibers. Terminate in pulmonary vesicles.
• Pulmonary Vesicles: Made up of alveoli.

Pulmonary Alveoli

• Site of air-blood contact, the thinness (0.2 to 0.5 um) allows efficient diffusion of gas. Two cell types:
• Pneumocytes I: Flattened cells forming the alveolar-capillary membrane (the respiratory exchanger).
• Pneumocytes II: Larger cells that secrete surfactant.
• Surfactant composition: 85% phospholipids, 5% neutral lipids and cholesterol, and 10% protein.
• Surfactant's role: reduces surface tension to facilitate alveolar dilation, prevents alveolar collapse, facilitates mucous flow, stimulates macrophage activity, and prevents desiccation.

Common Features of the Respiratory Tract

• Covered with vibratory cilia to trap and push dust outward.
• Mucus-secreting glands humidify the air, absorb dust, and eliminate pathogens.
• Vascularized and innervated to nourish and provide olfactory sensitivity.
• The respiratory tract supplies lungs with purified, humidified, and warmed air.

Lungs

• Soft, spongy, elastic organs divided into lobes: two on the left, three on the right, separated by the mediastinum.
• The base of the lungs rests on the diaphragm.
• Lungs covered by the pleura, which consists of a visceral sheet adhering to the lung and a parietal sheet lining the thoracic cage. The pleural fluid allows the two pleura to slide together.
• The pleura binds the lung to the thoracic cage, maintains negative pressure in the lung, and helps defend against inflammation and infection.
• CO2-rich blood enters the lungs via pulmonary arteries, branches into pulmonary capillaries, and becomes freed of CO2 and charged with O2. It drains into the left atrium. The lungs also receive feeder vessels.
• Primary function of the lungs is to breathe.
• Lungs are adapted to life in the air with a large exchange surface, ventilation for air renewal, and blood oxygen uptake, and also perform metabolic functions, transformation of angiotensin I into angiotensin II, eliminates, intercepts.

Breathing (External)

• Mechanical ventilation phenomena involve pressures:
• Atmospheric pressure (Patm): Constant at 760 mmHg (at sea level) and serves as the respiratory system reference (Patm = 0).
• Alveolar pressure (Palv): Pressure inside alveoli, varies during respiratory cycle, becomes equal to Patm at rest.
• Intrapleural pressure (Pip): Pressure inside the pleural cavity, varies and is always < Palv.
• The pressure difference between atmospheric and alveolar air (Patm - Palv) determines the movement of air. It consists of inspiration and expiration.
• The thoracic cage is a hermetically sealed muscular-bone compartment.
• Lung elasticity is due to elastic fibers, and the pleura connects the lungs to the thoracic cage.
• Intrapleural fluid (IPF) exerts a pressure known as intrapleural pressure (IPP). IPP is slightly lower than alveolar pressure (4 mm Hg).
• The difference, Transpulmonary Pressure (Palv - Pip = 4 mm Hg), determines the lung dimensions.

Boyle-Mariotte Law:

• The relationship between the pressure and volume of gases: At constant temperature and in an enclosed space, P is inversely proportional to V.

Quiet Breathing

• INSPIRATION: Active process where the ribcage expands under the inspiratory muscles, mainly the diaphragm.
• The rib-elevator muscles also lift.
• The decrease in Pip increases the pressure gradient across the Palv - Pip lung wall, causing the lungs to increase in volume. When Palv becomes lower than Patm, air flows into the lungs until the pressures equalize.
• EXIPRATION: Passive phenomenon that results from the return of the inspiratory muscles, which reduces the volume of the thoracic cage.
• Forced breathing: Involves maximum contraction of muscles leading to complementary air is taken in. During exhalation the abdominal and rib muscles contract. This allows for reserve air to be pushed out.

Factors influencing pulmonary ventilation

• Airways resistance, gas flow relates to pressure and resistance.
• Lung compliance (extensibility): Compliance is the ability of the lung to expand. The greater the compliance is, the easier it will be to expand the lung. Compliance depends on the elasticity of lung tissue, the rib cage, and surface tension in the alveoli.
• Surface tension in the alveoli is the tension is kept low by surfactant preventing alveoli from collapsing.
• Total duration of respiratory cycle: Adults at rest is 4 to 5 seconds; newborns at rest is 1 to 1.5 seconds. Formula for respiratory frequency (FR) is number of cycles per minute.
• Breathing volumes and capacities: Tidal volume: The amount of air inhaled at rest (500ml). At rest, the formula to calculate ventilatory flow (QV): 6 to 8 L.min¯¹ .

Air Changes During Breathing

• Volume of exhaled air appears greater than inhaled air because of produced heating in the respiratory tract. With temp inhaled ranging from 10 to 30C the exhaled air temp remains constant at around 35C. At 35C, exhaled air is saturated with water.

Chemical Changes

• The percentage composition of exhaled and inhaled air: O2 in inspired air is at 20.95% and exhaled air at 16.1%. CO2 in inspired air is at 0.03% and exhaled air at 4%. N is at 79.02% for both.
• Respiratory quotient: Ratio of the volume of CO2 released to the volume of O2 consumed. It varies according to the nature of the food. A diet rich in carbohydrates shows close to 1 RQ since glucose oxidation is expressed by the formula. If the diet is rich in lipid the RQ decreases towards 0.7.

Alveolar and Tissue Exchanges

• Hematosis: the process responsible for the transformation of venous blood arriving to the lungs into arterial blood flowing out.
• Gas exchange is determined by the difference in partial pressures of each gas in the two media separated by the permeable membrane. The diffusion of gas that flows through a membrane is governed by Fick's Law dV/dt = (S/E) x D x (P1-P2). Based on the law, diffusion is Proportional to tissue surface; inversely proportional to the thickness; Proportional to the concentration gradient; proportional to a constant. At the alveolar level: The partial pressure of oxygen is 105 mm Hg in alveolar air and 40 mm Hg in venous blood.

Oxygen

• Oxygen first dissolves in blood plasma. The hemoglobin becomes oxyhemoglobin. Transfer of oxygen from blood to the tissues will take place depending on its partial pressures.
• Oxygen delivery to tissues: Passage from blood to tissues is explained by the difference in partial pressures of this gas between arterial blood (105 mm Hg) and tissues (40 mm Hg).
• 3% of oxygen is dissolved in blood plasma and the rest is combined with hemoglobin. A red blood cell protein, which binds 02 at at the Fe(2+) site and can bind 4mol of O2. Partial pressure is the main O2 binding factor. Hemoglobin has a high affinity for (02) at the alveolar level and is known as saturated hemoglobin. Hemoglobin subunit engages in cooperative interactions.

Hemoglobin Saturation

• P50 = O2 pressure at which 50% of Hemoglobin is saturated with O2 (27 mmHg). High PO2 (pulmonary capillaries) O2. Low PO2 (tissue capillaries) 02 release.

Comparison of Hemoglobin and Myoglobin

• Myoglobin, present in muscle cells, is not a tetramer and is a monomer (a single unit that does not exhibit allosteric cooperation.

Factors influencing hemoglobin saturation

• Certain physiological situations can modify the oxyhemoglobin dissociation curve. The factors include increase or decrease in: Temperature, blood pH, C02 partial pressure, Blood concentration of 2,3-diphosphoglycerate.

Air

• Oxygen is delivered directly to the cells in gaseous form through a system of tubes called trachea. The trachea remains open through spiracle or stigma

The C02 cycle

• CO2 can combine with water to form carbonic acid. AC CO2 + H2O <-> H2CO3 <-> HCO3- + H+ 60% of CO2 is converted to hydrogen carbonate (HCO3).
• Hemoglobin can bind and release C02 known as carbamino-Hemoglobin and all blood proteins transport 30% of CO2.
• C02 automatism is constantly adapting to the changing needs of the organism regulated by: Humeral system: This system involves liquid such as cerebrospinal fluid and blood to send messages throughout the body such as levels of CO2. Nervous system involves the nerve that send messages back and forth.

Regulation of Breathing

• Breathing regulation occurs in the respiratory center the Medulla Oblongata composed of two groups of neurons. Impulses are transmitted to the respiratory muscles via efferent (centrifugal) nerves.
• The ventral breathing unit GRV is the respiratory generator center. The dorsal breathing group GRD integrates peripheral information and sends Efferences to GRV.
• The pneumotaxic center and the apneustic center are located in the pons and modulate the activity based on peripheral and central information. Inspiration is a active phenomenon , exhalation a passive.
• The nerve regulation reacts to the pain and increase respiratory rate.
• Chemical regulation directly influences chemoreceptors with blood composition.

Chemoreceptors

• Located on the aortic arch and the carotid artery.

• Peripheral C particularly sensitive to PaO2 variations, decreased in arterial P02 is a stimulus, increases the ventilation reflex. Not sensitive to P02.

• Central chemoreceptors located in the ventral surface of the medulla is stimulated by ions fluid . Formation results from the combination CO2 with H20, these will stimulate neuron and increased ventilation. H ions Mechanical control: From mechanoreceptors located in the lung parenchyma and muscles.

Environments and Respiration

Depending on whether the animal lives in air or water, the organization of respiratory exchange surfaces strongly influence. Where diffusion applies.

Living Environment Characteristics

Air

• is a oxygen-rich medium(209ml/l in air vs. 7ml/l in water)

Water

• Must dissolve; dissolution depends on solubility coefficient weak for oxygen, Temperature(solubility decreases with temperature), Quantity of ions contained in water which affects the level of saturation of temperature with liquid molecules.
• Ex. at 0°C saturation is at 14mg/L (freshwater) and 11mg/L at (seawater) and at 10°C saturation is at 10.9 mg/L It's easier to extract oxygen from air.
• The density and viscosity of water are higher than those of air.
• With aerial environment provides abundance and it is easier to mobilize unlike water . Oxygen consumption is lower for aquatic animals.

types of breathing-Depending on the degree of organism and environment

Aquatic breathing-Least efficient animals breathe with their integument and the system is used

• Invertebrate gills-folding of gills.

• Vertebrater gills- internal except in anuran tadpole. The gills are irrigated where the fish are able to extract 80% of the 02 dissolved in the water.

• Cutaneous respiration which is the only form in sponges.

• Breathing with air : Bird lungs are most efficient Birds require increased amounts of oxygen and a fixed thoraic is neded, The exchange of surface is more delevp and it extraction from 02 . There are tracheal in the form in insects and oxygen dirctly to the cells in form . There is also mixed version , the lung replaces gills, Low effiency.

The case of cetacean their tract , but they can close the of their voluntary during a dive.

Respiratory Pigments

Oxygen is taken up at the exchange surface by the blood, transporters are present in many animal organisms. Made of metaliproteins that helps binds oxygen and their change depending on redox. Hemin pigments made chains iron and protophyrin forms the heme (hemoglobilin, The second of heme is chlorocruonine Hemerythin pigments directly with poyl chains, color , violet with or w out 02 Also Hemocyanin pigments which are in case of metal an each that is in chain color blue or presence of , molecular is in in, arachinds d and mollsks